This study is concerned with the relationship of intracortical (cortico-cortical) synaptic connections to epileptogenesis and designed to test the following hypothesis: Based on previous investigations, both reviewed and from this laboratory, interruption of subcortico- cortical fibers (undercutting) will lower seizure threshold, but interruption of intracortical fibers will raise seizure threshold. 1. A subpial transverse cortical cut will be made in cat suprasylvian gyrus interrupting intracortical fibers and seizure threshold for electrically induced after discharge tested adjacent to this cut at varying times (in several animals) after the lesion and compared to intact and chronically undercut gyri in the same animal. A chronically implanted series is also planned to determine thresholds over time in the same animal. 2. Two parallel subpial cuts will be made in another series at varying distances of from 10 to 1 mm apart in suprasylvian, and seizure threshold tested within and outside this segment at varying times after the lesion. The series will be extended stepwise until complete cortical islands are made, of various sizes, to determine a "critical mass" for seizure. 3. Anatomical studies using selective silver impregnation methods and electronmicroscopy will be performed on animals with similar lesions to determine the origin, extent, and termination of the fiber systems involved.